Emerging evidence indicates that, in addition to endothelial cells, perivascular cells (pericytes) may constitute a relevant target cell type for antitumor angiogenesis therapy. In tumors such as glioblastoma multiforme, osteosarcoma, and fibrosarcoma, blood vessels are sparsely covered by pericytes. In contrast, we found that the vasculature of pancreatic cancer metastases in the liver is rich in pericytes that originate from hepatic stellate cells (HSC). HSC play a proangiogenic role for tumor metastasis. Thus, inhibition of HSC migration may be beneficial for suppressing liver tumor angiogenesis. Our preliminary studies demonstrated that in both a metastatic and genetic model of primary liver tumors, Nitric oxide (NO) inhibits HSC recruitment in tumors thereby limiting tumor growth. In vitro, NO inhibits PDGF-induced HSC proliferation and migration, and NO inhibits HSC migration via inhibition of Rho family small GTPase Rac1-driven filopodia formation. These observations led us to formulate the central hypothesis of this proposal: NO impairs HSC recruitment in liver tumors by inhibition of Rac1-driven migratory signals. The specific aims of this proposal are: 1) to determine how NO inhibits Rac1-driven filopodia formation and migration through perturbation of Rac1 - IQGAP1 - N-cadherin/catenin/actin signaling pathway;and 2) to determine how NO impairs pericyte recruitment in liver metastases through a pathway involving Rac1 and N-cadherin. In Aim I, we will use in vitro experiments with human HSC to explore the role of NO in the regulation of downstream signals of Rac1, such as Rac1/IQGAP1 complex and N-cadherin/catenin/actin complex. In Aim II, we will co-implant tumor cells with exogenous pericytes that express Rac1 transgenes to elucidate the interplay of NO and Rac1 in the regulation of pericyte recruitment to tumors. Additionally, we will treat tumor-bearing mice with neutralizing anti- N-cadherin antibody (GC-4 mAb) to characterize the role of N-cadherin in pericyte function in vivo. Furthermore, micro-CT will be utilized to analyze the tumor vascular network in a genetic model of liver tumors to determine whether perturbation of HSC function by NO impacts the tumor vasculature. Thus this proposal will utilize innovative approaches to delineate novel regulatory mechanisms of NO in the modulation of pericyte motility and tumor angiogenesis, and thereby further advance our understanding of tumor pericyte biology and NO function.